Thermal stability, silenes

Silenes formed in direction A can exothermically be dimerized to form dimers of two types (head-to-tail and head-to-head), which differ in thermal stability (Scheme 40). The energy parameters of these reactions are presented in Table XVIII. 

Matrix IR spectra of various silenes are important analytical features and allow detection of these intermediates in very complex reaction mixtures. Thus, the vibrational frequencies of Me2Si=CH2 were used in the study of the pyrolysis mechanism of allyltrimethylsilane [120] (Mal tsev et al., 1983). It was found that two pathways occur simultaneously for this reaction (Scheme 6). On the one hand, thermal destruction of the silane [120] results in formation of propylene and silene [117] (retroene reaction) on the other hand, homolytic cleavage of the Si—C bond leads to the generation of free allyl and trimethylsilyl radicals. While both the silene [117] and allyl radical [115] were stabilized and detected in the argon matrix, the radical SiMc3 was unstable under the pyrolysis conditions and decomposed to form low-molecular products. 

The potential surface for die gradient path addition of ethylene to silene and the possible existence and stability of intermediates in the thermal decomposition reaction of silacyclobutane has been explored.38 The energy maximum of die multi-step process corresponds to a cyclic transition state leading on one side to a planar silacyclobutane transition state which falls to ground-state puckered silacyclobutane and on the other side to a trans diradical which fragments to ethylene and silene. 

Silenes (compounds with an Si=C double bond) are known to be extremely reactive compounds. Till now, only a few silenes, stabilized kinetically by bulky substituents, could be isolated under normal conditions and were characterized by X-ray structural analyses [1]. An effective stabilization of Si=C systems is also achieved by coordination of a base, such as amines, THF, or fluoride ions, to the electrophilic silene silicon atom [2], and recendy we succeeded in synthesizing thermally unexpectedly stable intramolecularly donor-stabilized silenes [3]. In this paper, the synthesis and, in particular, the behavior of the l-[2,6-bis(dimethylaminomethyl)phenyl]silenes 2a-d are described. 

The intramolecularly donor-stabilized silenes 2a-d are thermally stable compounds. 2a, 2c and 2d are colorless solids 2b is an oil. X-ray structural analyses unambiguously revealed that in the solid state only one dimethylamino group of the ligand is coordinated to the silene silicon atom. Important structural parameters, e.g., the Si=C bond lengths and the Si-N distances, are in good agreement with data obtained for other intramolecularly amine-coordinated silenes [3]. 

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